Numerical simulation of a membrane desorber with the H2O-LiBr working mixture for absorption cooling systems

Abstract

In absorption cooling systems, the refrigerant fluid is separated from the working-fluid mixture inside a component called “desorber”. Conventional desorbers operate by boiling separation; however, membrane modules are a potential alternative to replace them since they can operate below the working mixture boiling point and at atmospheric pressure conditions. The present paper presents a numerical simulation of a membrane desorber that uses the air gap membrane distillation configuration. The numerical analysis was carried out by means of ANSYS FLUENT CFD code using a 3D model. Temperature, concentration, and velocity contours were numerically obtained assuming H2O-LiBr solution at 41 % w/w, mass flow of 0.03 kg/s, temperature of 363.15 K, and cooling water at 303.15 K and 0.04 kg/s. The maximum error in simulated temperature was 11.9 % compared to the experimental data. According to the results, a solution temperature difference between the bulk and the membrane interphase up to 288.15 K was calculated. In addition, stagnated areas inside the solution channel cause velocity differences of up to 5 magnitude orders compared to the middle point in the solution channel. Since a maldistribution flow causes a “jet” inside the solution channel, and a non-homogeneous concentration distribution on the membrane interphase; as a result, a concentration difference up to 1.35 % between a point located at the border membrane interphase compared to a point located at the middle was observed. Therefore, the desorption rate can be improved with geometric modifications in the membrane desorber device.